At present, a hard disk is generally a thin film disk utilizing the longitudinal recording technology. The magnetization direction of the magnetic recording medium is in a longitudinal arrangement. The data read/write is achieved by use of a magnetic head or heads which slide(s) on the surface of the thin film, and utilizes a coil for dealing with a magnetic moment induction, or for generating a magnetic field by connecting to a current source in order to alter the magnetization direction of the magnetic moment. The hard disk arranged in such a manner is used in the so-called Winchester disk drive, and plays an important role on the commercial hard disk market. With greatly and rapidly progressing in technologies, new thin film disk technologies are proposed one after another. For example, the perpendicular recording and magneto-optic recording technologies which both have the advantage of large recording capacity attract many attentions in this field, and threaten the market of the longitudinal recording technology of the conventional hard disk. In addition, the trends of being more light, thinner, shorter, and smaller in technical products also push the makers to increase the recording capacity of the conventional hard disks.
In order to act as a longitudinal magnetic recording thin film having a high recording density, the following requirements should be meeted with: 1. The magnetic recording material must have a high coercivity (Hc) at least higher than 1000 Oe; 2. The residual magnetization thereof (Mr) has to be sufficiently high; 3. The thickness of the magnetic thin film has to be decreased in order to lower the demagnetization interaction among the magnetic areas of the thin film; and 4. The thin film must have a high signal to noise ratio (SNR). The ferrite magnetic thin film has a better SNR than the metal magnetic thin film has, and its coercivity can be higher than 1000 Oe by adding cobalt (Co) and manganese (Mn), and by using appropriate heat treatment conditions. Therefore, the ferrite magnetic thin film is known as an excellent material for the longitudinal magnetic thin film having a high recording density.
The paper: E. T. Wuori and D. E. Speliotis, "Plating in the Electronic Industry", 3rd symposium, (1971), 315, disclosed an iron thin film formed by an evaporation method. The evaporated iron thin film is converted into an .alpha.-Fe.sub.2 O.sub.3 film by an oxidation treatment at temperature about 450.degree.-500.degree. C., and is then reduced to an Fe.sub.3 O.sub.4 film in an H.sub.2 and CO atmosphere. Another paper: R. L. Comstock and E. B. Moore, IBM J. Res. Dev. 18, (1974), 55, disclosed that an aqueous solution of Fe(NO.sub.3).sub.3 9H.sub.2 O is sprayed to a preheated substrate at 300.degree.-350.degree. C. by a rotation deposition method to obtain an .alpha.-Fe.sub.2 O.sub.3 thin film, and then the Fe.sub.3 O.sub.4 and .gamma.-Fe.sub.2 O.sub.3 thin films can be formed respectively by reduction and oxidation heat-treatments.
Prior researchers all concentrated on the phase synthesis, and did not dope any additives. Consequently, the resultant coercivity cannot be high, and can only reach about 200 Oe. Since 1977, some researchers proposed to utilize the technology of doping cobalt in the .gamma.-Fe.sub.2 O.sub.3 magnetic particulates to increase the coercivity of the formed thin film. The paper: M. Satuo et al., IEEE Trans. Magn., MAC-13, (1977), 1400, proposed an evaporation method for preparing a Co.sub.x Fe.sub.3-X O.sub.4 thin film. In this method, iron is first evaporated on a glass substrate and converted into .alpha.-Fe.sub.2 O.sub.3 phase by oxidation at 400.degree. C. Then, cobalt is evaporated thereon, and diffused into the .alpha.-Fe.sub.2 O.sub.3 film to form the Co.sub.x Fe.sub.3-x O.sub.4 thin film by an annealing process in a vacuum. The coercivity of the resulting thin film is enhanced up to 700 Oe.
The enhancement of coercivity is so large that many researchers join in this field. The paper: S. Hattori et al., IEEE Trans. Magn., MAG-15, (1979), 1549, first proposed a reactive sputtering method which is able to successfully prepare an iron-oxide magnetic thin film with additives of Co and Ti. .alpha.-Fe.sub.2 O.sub.3 is first deposited in an Ar-O.sub.2 atmosphere, reduced to Fe.sub.3 O.sub.4 in an H.sub.2 atmosphere, and then oxidized to .gamma.-Fe.sub.2 O.sub.3. The coercivity of the obtained thin film is about 700 Oe, the squareness ratio is about 0.8, and the recording density is about 1100 bits/mm. This preparing method is the most popular process for manufacturing the iron-oxide series hard disk in industry.
Next year, the paper: S. Hattori et al., IEEE Trans. Magn., MAG-15, (1980), 1114, disclosed that the same researcher group as above further improvedly omitted one step of the preparing process. Fe.sub.3 O.sub.4 is synthesized directly by a reactive sputtering method, and then is oxidized to obtain the .gamma.-Fe.sub.2 O.sub.3. Its squareness ratio is increased to about 0.85, but its coercivity is slightly decreased to about 600 Oe. Therefore, the subsequent researchers seldom adopt this process while preferring to the above method (disclosed in 1979).
In 1984, O. Ishii and I. Hatakeyama in the above researcher group proposed, as disclosed in the paper: J. Appl. Phys. 55, (1984), 2269, a reactive sputtering method to dope osmium (Os) into a ferrite magnetic thin film, and obtained a great break-through. They found that the doped Os can impart to the thin film several advantages of: 1. largely increasing the coercivity to about 1920 Oe which cannot be reached by only doping the modifier Co; 2. being capable of efficiently suppressing the grain growth during later heat treatments so as to increase the SNR of the thin film, like in the case of doping Cu; 3. enhancing the coercivity up to about 2100 Oe after the annealing in a magnetic field (7 kOe); and 4. increasing the coercive squareness S* up to about 0.95, which is more efficient than that achieved by adding Ti. However, the worst drawback of this method is that the osmium is too expensive. Therefore, a strong desire to further improve the ferrite magnetic thin film still exists in this field.
Most of the above-mentioned papers typically utilize the vacuum deposition manner to prepare the ferrite magnetic thin film. However, their necessary equipments are very expensive. Another spray pyrolysis method is also practicable, and requires only simple and economical equipments. The paper: M. Langlet et al., IEEE Trans, Magn., MAG-22, (1986), 151, disclosed such a spray pyrolysis method which is able to prepare a ferrite magnetic thin film having a high coercivity. An iron acetylacetonate is dissolved in a butanol, and an ultra-sonic vibrator utilized to aerosolize the solution. A mixed gas of Ar-O.sub.2 is used to blow the aerosol droplets to the surface of a glass substrate at about 420.degree.-550.degree. C. to form a .gamma.-Fe.sub.2 O.sub.3 -Fe.sub.3 O.sub.4 thin film. The resultant film after adequate heat treatments has a maximum coercivity of about 800 Oe, and an Ms of about 400 emu/cm.sup.3. If 10 wt% Co is doped therein the coercivity of the film can be increased to more than about 1000 Oe, and the Ms is about 350 emu/cm.sup.3.
From the above descriptions, it is noted that the conventional methods have several drawbacks: 1. The equipments needed by the vacuum deposition method are very expensive. 2. It is difficult for the vacuum deposition method to achieve a mass or continuous production. 3. If an expensive metal, such as Os, is doped to enhance the film coercivity, the film cost will be too high.